This invention relates to the manufacture of a disc suitable for recording of information such as video information. More particularly, this invention relates to a method of producing a groove in the surface of a disc prior to recording video information therein.
A typical system for recording and playing back of video information has been described by Jon K. Clemens in copending application Ser. No. 126,772 filed Mar. 22, 1971, now U.S. Pat. No. 3,842,194, which is assigned to the same assignee as the present invention. According to this system, an aluminum disc is first coated with a lacquer and a spiral groove is cut therein. The grooved lacquer surface is thereafter replicated by producing negative and then positive reproductions thereof by a series of nickel deposition steps. A photoresist coating is then applied to the positive replicated nickel. Video information is then recorded by selectively exposing the photoresist in the spiral groove to a video-signal-modulated electron beam produced by a scanning electron microscope. After exposure and development of the photoresist, the video information appears on the bottom and wall regions of the spiral groove in the form of geometric or topographical variations. This disc is then replicated by metal plating and the plated replica thereafter is used to stamp or emboss vinyl discs by methods known in the audio recording art. The vinyl replica is then metalized to form a conducting surface, which, in turn, is coated with a dielectric material. In playing back the recorded information, a stylus is caused to ride in the dielectric coated groove. This stylus, along with the metalization and dielectric, acts as a capacitor. Capacitance variations in the groove, which correspond to the recorded video information, are then detected electronically to recover the video information.
Groove pitch in such a video disc generally is between 2000 and 8000 groove convolutions per inch. As the number of groove convolutions per inch on a disc is increased, the cross-sectional dimensions available for recording in the groove decrease. That is, the amount of usable wall and valley region available in the cut groove for recording video information decreases. A decrease in the distance measured along the curved surface of the groove from the top (peak) of one wall to the top (peak) of the other wall (hereinafter referred to as the "surface width" of the groove) results in a decrease in the detectable variation in capacitance which manifests in a decreased signal output during playback of the embossed disc. In order to maintain a high signal-to-noise ratio in the playback of a replicated or embossed disc, it is desirable that the maximum usable surface width of the groove be utilized for recording video information and that such surface width also be contacted by the stylus in playing back the information. The greater the surface contact of the stylus with the video disc, the greater will be the detectable capacitance and its variations and the greater the signal-to-noise ratio.
In prior systems, the surface of the photoresist-coated groove prior to exposure to the scanning electron microscope has a substantially sinusoidal cross-sectional shape. This shape results from the close spacing of adjacent V-shaped groove portions or convolutions and the method of depositing photoresist thereon previously used. Such a sinusoidal shape has inflection points at or near its median amplitude, providing thereby a concave-shaped curve below the median level and a convexshaped curve above the median level. This curvature, upon which the video-information-carrying variations are impressed, should be taken into account in the shaping of a playback stylus. That is, if the groove shape is substantially sinusoidal in cross-section, a stylus placed in that groove preferably would have a complementary sinusoidal shape to make contact with a maximum wall and valley region of the groove. Typically the stylus tip is more nearly of a constant radius of curvature (i.e., a portion of a circle). To achieve this sinusoidal stylus shape in the stylus requires an additional manufacturing step of lapping the stylus tip to conform with the sinusoidal groove shape.
A further disadvantage of having a stylus tip that is not of substantially constant radius of curvature occurs if the groove in the disc is eccentric or if the video pickup arm housing the stylus is not perfectly centered over the groove. Under such conditions the stylus does not maintain perpendicularly with the groove and the contacted area between stylus and groove changes for small deviations in angle between stylus and disc surface. This change in contacted area undesirably results in a change in signal-to-noise ratio.
It is advantageous to shape the surface of the groove in the disc such that the stylus can more easily be made to conform with it and therefore utilize wall and valley regions of the groove for recording of information.